Data recording medium with servo pattern having pseudo-noise sequences

ABSTRACT

A data recording medium has tracks with pseudo-noise (PN) sequences with good autocorrelation properties as servo information for controlling the position of the recording head. A first set of alternating tracks uses a leading pseudo-random binary sequence (PRBS), which is a PN sequence with good autocorrelation properties, and a following PRBS that is cyclically shifted from the leading PRBS. A second set of alternating tracks interleaved with the first set also has a leading PRBS and a following PRBS that is cyclically shifted from the leading PRBS, but the leading PRBS in each of the tracks in the second set is offset along-the-track from the leading PRBS in the tracks of the first set. The head positioning control system uses the leading PRBS to generate a servo timing mark (STM), the cyclic shift to generate track identification (TID), and the following PRBS from adjacent tracks to generate the head position error signal (PES).

RELATED APPLICATION

This application is related to concurrently filed application Ser. No.10/841,034, filed May 6, 2004, now U.S. Pat. No. 6,995,938B2, and titledDATA RECORDING SYSTEM WITH SERVO PATTERN HAVING PSEUDO-NOISE SEQUENCES.Both applications are based on a common specification, with thisapplication having claims directed to a data recording medium andapplication Ser. No. 10/841,034 having claims directed to a datarecording system.

TECHNICAL FIELD

This invention relates generally to data recording systems, such asmagnetic recording hard disk drives, and more particularly topre-recorded servo patterns and servo positioning systems to locate andmaintain the read/write heads on the data tracks.

BACKGROUND OF THE INVENTION

Magnetic recording hard disk drives use a servo-mechanical positioningsystem to hold the read/write head on the desired data track and to seekfrom track to track as required to perform read and write operations.Special “servo” information is written in fields incircumferentially-spaced servo sectors in each of the concentric datatracks on each disk surface. The servo pattern is constructed acrossmultiple tracks so that the read-back signal from the head, as it passesover the pattern, can be decoded to yield the radial position of thehead. The servo pattern is written onto the disk during manufacturing ina process known as servowriting.

In conventional servowriting the servo pattern is written in multiplepasses using the regular write head in conjunction with a specializedservowriter. Each pass must be precisely aligned circumferentially.Misalignment introduces errors into the servo system. As the density ofthe tracks in the radial direction and the linear density of the databits in the circumferential or along-track direction increase it becomesincreasingly difficult to precisely align the servo fieldscircumferentially.

What is needed is a magnetic recording disk having a servo pattern, anda disk drive having a servo decoding system, that are not sensitive tomisalignment of the pre-recorded servo fields.

SUMMARY OF THE INVENTION

The invention is a data recording medium in which the tracks havepseudo-noise (PN) sequences with good autocorrelation properties andcapable of detection by a correlation filter as servo information. A PNsequence can be used to encode a servo timing mark (STM) to identify thestart of the servo pattern in each of the tracks. A servo pattern with aPN sequence in each track, where the PN sequence is shifted along-thetrack from the PN sequence in adjacent tracks, can be decoded by thecontrol system for track identification (TID) and for the position errorsignal (PES) representative of the position of the head between thetracks.

In one embodiment, a specific type of PN sequence called a pseudo-randombinary sequence (PRBS) is used. A first set of alternating tracks has aleading PRBS and a following PRBS that is cyclically shifted from theleading PRBS. A second set of alternating tracks interleaved with thefirst set also has a leading PRBS and a following PRBS that iscyclically shifted from the leading PRBS, but the leading PRBS in eachof the tracks in the second set is offset along-the-track from theleading PRBS in the tracks of the first set. The first or leading PRBSin the tracks is used by the head positioning control system as a STM.The cyclic shift increases by a fixed increment across the tracks and isused by the control system for the TID. The following PRBS in the tracksis used by the control system as the PES.

In a magnetic recording disk drive implementation of the invention, theservo pattern is in angularly-spaced servo sectors and the PRBS used inthe sectors in the second set of alternating tracks is the inverse ofthe PRBS used in the sectors in the first set of alternating tracks. Thedisk drive includes a correlator that outputs a single positive dipulseeach time a PRBS from the first set of tracks is detected and a singlenegative dipulse each time an inverted PRBS is detected. Thezero-crossing of the dipulse from detection of the first or leading PRBSin each track is used as timing to generate a STM. The cyclic shiftbetween the leading and following PRBS increases by a fixed incrementwith each track in the radial direction so that the length of the cyclicshift between the leading and following PRBS in each track representsthe TID. The difference in amplitude of the dipulses from detection ofthe following PRBS in two adjacent tracks represents the PES sent to thedisk drive actuator to maintain the head on track.

For a fuller understanding of the nature and advantages of the presentinvention, reference should be made to the following detaileddescription taken together with the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of a prior art disk drive of the type usablewith the present invention.

FIG. 2A is a portion of a typical data track on the disk of the diskdrive shown in FIG. 1.

FIG. 2B is an expanded view of one of the servo sectors in the datatrack of FIG. 2A.

FIG. 3 is a block diagram of the servo electronics in the prior art diskdrive in FIG. 1.

FIG. 4A is a prior art servo pattern with a quad-burst PES pattern.

FIG. 4B shows the effect of circumferential misalignment on the priorart servo pattern in FIG. 4A.

FIG. 5A is the servo pattern of the present invention

FIG. 5B is a pseudo-random binary sequence (PRBS) for the servo patternin FIG. 5A.

FIG. 6 is a block diagram of the servo decoder of the present invention.

FIG. 7 shows the output of the correlator and how it is used to detectthe servo timing mark (STM) for the servo pattern in FIG. 5A.

FIG. 8 shows the output of the correlator and how it is used to decodethe track identification (TID) field for the servo pattern in FIG. 5A.

FIG. 9 shows the output of the correlator and how it is used to decodethe position error signal (PES) field for the servo pattern in FIG. 5A.

FIG. 10 is a diagram of a linear feed-back shift register (LFSR)commonly used to generate a PRBS.

DETAILED DESCRIPTION OF THE INVENTION

Description of the Prior Art

FIG. 1 is a block diagram of a disk drive of the type usable with thepresent invention. The disk drive depicted is one that is formattedusing a fixed-block “headerless” architecture with sector servo andzone-bit recording (ZBR).

The disk drive, designated generally as 102, includes data recordingdisk 104, actuator arm 106, data recording transducer 108 (also called ahead, recording head or read/write head), voice coil motor 110, servoelectronics 112, read/write electronics 113, interface electronics 114,controller electronics 115, microprocessor 116, and RAM 117. Therecording head 108 may be an inductive read/write head or a combinationof an inductive write head with a magnetoresistive read head. Typically,there are multiple disks stacked on a hub that is rotated by a diskmotor, with a separate recording head associated with each surface ofeach disk. Data recording disk 104 has a center of rotation 111 and isrotated in direction 130. Disk 104 is divided for head positioningpurposes into a set of radially-spaced concentric tracks, one of whichis shown as track 118. The tracks are grouped radially into a number ofzones, three of which are shown as zones 151, 152 and 153. Each trackincludes a plurality of circumferentially or angularly-spaced servosectors. The servo sectors in each track are aligned circumferentiallywith the servo sectors in the other tracks so that they extend acrossthe tracks in a generally radial direction, as represented byradially-directed servo sections 120. Each track has a reference index121 indicating the start of track. Within each zone, the tracks are alsocircumferentially divided into a number of data sectors 154 where userdata is stored. The data sectors contain no data sector identification(ID) fields for uniquely identifying the data sectors so the drive isconsidered to have a “No-ID”™ type of data architecture, also called a“headerless” data architecture. If the disk drive has multiple heads,then the set of tracks which are at the same radius on all disk datasurfaces is referred to as a “cylinder”.

Read/write electronics 113 receives signals from head 108, passes servoinformation from the servo sectors to servo electronics 112, and passesdata signals to controller electronics 115. Servo electronics 112 usesthe servo information to produce a current at 140 which drives voicecoil motor 110 to position head 108. Interface electronics 114communicates with a host system (not shown) over interface 162, passingdata and command information. Interface electronics 114 alsocommunicates with controller electronics 115 over interface 164.Microprocessor 116 communicates with the various other disk driveelectronics over interface 170.

In the operation of disk drive 102, interface electronics 114 receives arequest for reading from or writing to data sectors 154 over interface162. Controller electronics 115 receives a list of requested datasectors from interface electronics 114 and converts them into zone,cylinder, head, and data sector numbers which uniquely identify thelocation of the desired data sectors. The head and cylinder informationare passed to servo electronics 112, which positions head 108 over theappropriate data sector on the appropriate cylinder. If the cylindernumber provided to servo electronics 112 is not the same as the cylindernumber over which head 108 is presently positioned, servo electronics112 first executes a seek operation to reposition head 108 over theappropriate cylinder.

Once servo electronics 112 has positioned head 108 over the appropriatecylinder, servo electronics 112 begins executing sector computations tolocate and identify the desired data sector. As servo sectors pass underhead 108, the headerless architecture technique identifies each servosector. In brief, a servo timing mark (STM) is used to locate servosectors, and a count of STMs from a servo sector containing an indexmark 121 uniquely identifies each servo sector. Additional informationis maintained in association with servo electronics 112 and controllerelectronics 115 for controlling the reading or writing of data in thedata sectors.

In FIG. 2A a portion of a typical track 118 on the disk 104 is shownexpanded. Four complete data sectors are shown (201, 202, 203 and 204).Three representative servo sectors 210, 211, and 212 are also shown. Ascan be seen from this example, some data sectors are split by servosectors, and some data sectors do not start immediately following aservo sector. For example, data sectors 202 and 204 are split by servosectors 211 and 212, respectively. Data sector 202 is split into datasections 221 and 222, and data sector 204 is split into data sections224 and 225. Data sector 203 starts immediately after the end of datasector 202, rather than immediately following a servo sector. The indexmark 121 indicates the beginning of the track and is shown contained inservo sector 210.

FIG. 2B is an expanded view of one of the servo sectors illustrated inFIG. 2A. Typically, each servo sector contains an STM 306. The STM 306serves as a timing reference for reading the subsequent servoinformation in track identification (TID) field 304 and position errorsignal (PES) field 305. The STM is sometimes also referred to as a servoaddress mark or servo start mark. Each servo sector also contains anautomatic gain control (AGC) field 302 for controlling a variable gainamplifier (VGA) that adjusts the strength of the signal read by head108.

FIG. 3 is a block diagram of the servo electronics 112. In operation,controller electronics 115 provides input to actuator position control404, which in turn provides a signal 140 to the actuator to position thehead. The controller electronics 115 uses the servo information readfrom the servo sectors to determine the input 428 to the actuatorposition control 404. The servo information is read by the read/writeelectronics 113 (FIG. 1), and signals 166 are input to the servoelectronics 112. STM decoder 400 receives a clocked data stream 166 asinput from the read/write electronics 113, and a control input 430 fromthe controller electronics 115. Once an STM has been detected, an STMfound signal 420 is generated. The STM found signal 420 is used toadjust timing circuit 401, which controls the operating sequence for theremainder of the servo sector.

After detection of an STM, the track identification (TID) decoder 402receives timing information 422 from timing circuit 401, reads theclocked data stream 166, which is typically Gray-code encoded, and thenpasses the decoded TID information 424 to controller electronics 115.Subsequently, PES decode circuit 403 captures the PES signal fromread/write electronics 166, then passes position information 426 tocontroller electronics 115. Inputs to the PES decode circuit 403 aretypically analog, although they may be digital or of any other type. ThePES decode circuit 403 need not reside within the servo electronicsmodule 112.

FIG. 4A is a schematic of a conventional servo pattern of the typecommonly used in sector servo systems and shows a greatly simplifiedpattern for clarity with only four tracks (tracks 308, 309, 310 and 311having track centerlines 328, 329, 330 and 331, respectively). The servopattern moves relative to head 108 in the direction shown by arrow 130.The two possible magnetic states of the medium are indicated as blackand white regions. FIG. 4A shows the servo pattern in only fourradially-adjacent servo sectors in one servo section 120 of the disk,but the pattern extends radially through all the data tracks in eachservo section 120.

The servo pattern is comprised of four distinct fields: AGC field 302,STM field 306, Track ID field 304 and PES field 305. The servopositioning information in PES field 305 is a conventional quad-burstpattern comprising bursts A–D. The automatic gain control (AGC) field302 is a regular series of transitions and is nominally the same at allradial positions. The AGC field 302 allows the servo controller tocalibrate timing and gain parameters for later fields. The STM field 306is the same at all radial positions. The STM pattern is chosen such thatit does not occur elsewhere in the servo pattern and does not occur inthe data records. The STM is used to locate the end of the AGC field andto help locate the servo pattern when the disk drive is initialized. TheTID field 304 contains the track number, usually Gray-coded and writtenas the presence or absence of recorded dibits. The TID field 304determines the integer part of the radial position. The position errorsignal (PES) bursts A–D are used to determine the fractional part of theradial position. Each PES burst comprises a series of regularly spacedmagnetic transitions, the transitions being represented by thetransitions between the black and white regions in FIG. 4A. The PESbursts are arranged radially such that a burst of transitions are onetrack wide and two tracks apart, from centerline to centerline. PESbursts are offset from their neighbors such that when the head iscentered over an even-numbered track (e.g., track 310 with centerline330) the read-back signal from burst A is maximized, the read-backsignal from burst B is minimized and the read-back signal from bursts Cand D are equal. As the head moves off-track in one direction theread-back signal from burst C increases and the read-back signal fromburst D decreases until, with the head half-way between tracks theread-back signal from burst C is maximized, read-back signal from burstD is minimized and read-back signals from bursts A and B are equal. Asthe head continues to move in the same direction the read-back signalfrom burst B increases and the read-back signal from burst A decreasesuntil, with the head centered over the next track (with an odd tracknumber, e.g. track 311 with centerline 331) the read-back signal fromburst B is maximized, the read-back signal from burst A is minimized andthe read-back from signals from bursts C and D are again equal.

The prior art servo pattern shown in FIG. 4A is written track-by-trackwith a regular write head. Alignment of each individual track with itsneighbors is a key problem in writing the servo pattern. Two distinctalignment problems may occur. Track misregistration (TMR) occurs due toan error in the radial position of the head during servowriting. Thistranslates to a repeatable error in the servo position informationobtained from the servo pattern. Circumferential or along-trackmisalignment occurs due to an error in the circumferential position ofthe head during servowriting. Circumferential misalignment causesfeatures which span more than one track to become irregular anddistorted. FIG. 4B shows the effect of circumferential misalignment 312on the servo pattern shown in FIG. 4A. In practice circumferentialmisalignment must be much smaller than the smallest circumferentialfeature in the servo pattern. As the recording density increases theservo pattern features become correspondingly smaller andcircumferential misalignment becomes more of a problem.

The effect of circumferential misalignment is most pronounced where thehead is reading significant contributions from features written ondifferent tracks. For example, as shown in FIG. 4B, when the head ispositioned mid-way between track centerline 328 and track centerline 329the AGC field 302 contributions from the two tracks interferedestructively.

DESCRIPTION OF THE INVENTION

The invention will be described with respect to a magnetic recordinghard disk drive implementation, but the invention is applicable ingeneral to data recording systems that have data recorded in adjacentdata tracks that also include servo information for positioning the datarecording head or transducer.

The invention will be described using pseudo-random binary sequences(PRBS), but applies to the use of other pseudo-noise (PN) sequences. Inthe context of this invention, a PN sequence is any sequence withapproximately noise-like autocorrelation properties suitable fordetection by correlation filters. A PRBS is a specific type of PNsequence having very good autocorrelation properties, making it a goodchoice for the described embodiment.

FIG. 5A shows the servo pattern of the present invention. The AGC, STM,TID, and PES fields in the prior art are replaced by a pair PRBS fields.The PRBS fields in each servo track are identical, but the PRBS fieldsin adjacent tracks are different. The first PRBS field in alternatetracks (e.g., the odd servo tracks) are offset from the first PRBS fieldin the other tracks (i.e., the even servo tracks). The first or leadingPRBS field on a servo track serves as the STM. The STM provides areference for windowing the second PRBS field which is used to encodeboth the TID and the PES. The TID is encoded in the circumferentialphase relationship due to a cyclic shift between the first PRBS fieldand the second PRBS field. The PES is derived from the relativecontributions of the second or following PRBS field from the differentservo tracks as the read head crosses adjacent servo tracks in theradial direction. The PRBS fields provide an effective method fordealing with circumferential misalignment because the PRBS fields onadjacent tracks are not required to be precisely phase aligned.

The properties of a PRBS, the method of generating a PRBS, and theconcept of correlation are well-known and described extensively in thetechnical literature, for example see MacWilliams and Sloane,Proceedings of the IEEE, VOL. 64, NO. 12, pp 1715–1729.

The correlation of two sequences a(t) and b(t) is defined as:

${R_{a,b}(\tau)} = {\sum\limits_{t}{{a(t)}{b\left( {\tau + t} \right)}}}$This definition of correlation is well-known in the field of signalprocessing and is very similar to the statistical definition ofcorrelation:

${R_{a,b}(\tau)} = {{E\left\lbrack {{a(t)}{b\left( {\tau + t} \right)}} \right\rbrack} = {\lim\limits_{N->\infty}{\frac{1}{N}{\sum\limits_{t = 0}^{N - 1}{{a(t)}{b\left( {\tau + t} \right)}}}}}}$In both cases the quantity τ is known as the “lag” between sequences aand b. The correlation sum given above is very similar to theconvolution sum and it can be shown that the correlation of a(t) withb(t) is equal to the convolution of a(t) with b(−t). As a corollary ofthis, the correlation of an input sequence a(t) with a fixed referencesequence b(t) can be obtained using a filter with impulse responseb(−t). A filter of this sort is referred to as a correlator matched tosequence b(t).

A pseudo-random binary sequence (PRBS), also called a maximal-lengthshift-register sequence (M sequence), is a periodic sequence of binarybits with a number of interesting properties. In particular, theautocorrelation function of an N-bit PRBS, that is, the correlation ofan N-bit PRBS pattern with itself, is 1 for zero lag and 1/N elsewhere,up to lag N (whereupon it repeats). This is the property that givespseudo-random binary sequences their name since a sequence of purelyrandom binary bits would have an autocorrelation 1 at zero lag andautocorrelation 0 elsewhere. A direct consequence of this property isthat if a periodic PRBS is input to a correlator matched to a singleperiod of the same PRBS, the correlator will output a single narrowpulse each time the PRBS repeats. If a periodic PRBS is recorded using amagnetic recording system and the resulting read-back signal input to amatched correlator the correlator will output the dipulse response ofthe magnetic recording system each time the PRBS repeats. For afinite-length (i.e., not repeating indefinitely) PRBS the correlatoroutput will be valid after one full period has been input to thecorrelator, and will remain valid until the last sample of the PRBS hasbeen input to the correlator.

A PRBS can be generated using a linear feedback shift register (LFSR) inwhich the feedback polynomial is primitive. A PRBS is typically 2^(n)−1bits long where n is an integer. FIG. 10 is an example of a LFSR with 5latches that implement a 5^(th) order polynomial used to generate a31-bit PRBS. For a 5^(th) order polynomial there exist 6 primitivepolynomials that will produce a PRBS. In the preferred embodimentdescribed here two PRBS are used. The two sequences are formed by takinga PRBS and that PRBS inverted, i.e., the PRBS with each 1 inverted to a0 and each 0 inverted to a 1. The inverted sequence is also a PRBS has adipulse response with polarity opposite that of the original sequencewhen input to a correlator matched to the original sequence. When theinverted PRBS is input to the correlator matched to the original PRBS,there exists a range over which there is zero correlation. Over thisrange of lag values the two sequences are said to be orthogonal. Thecorrelator is a digital finite-impulse-response (FIR) filter matched tothe PRBS. The correlator is matched in the sense that the impulseresponse of the filter h[k] is equal to one period of the time-reversedPRBS, that ish[k]=x[n−k] k=0, 1, . . . n−1.A consequence of the autocorrelation property of pseudo-random sequencesis that when a PRBS is input to a matched correlator, the output iseither 1 or −1/n.

In FIG. 5A, the patterns labeled +PRBS denote the original sequence andthe patterns labeled −PRBS denote the inverted sequence. FIG. 5B showsan example of the recorded magnetization patterns from the original PRBS(+PRBS) and the inverted PRBS (−PRBS), wherein the dark regionsrepresent positive (“north”) magnetizations and the light regionsrepresent negative (“south”) magnetizations. The +PRBS is used in oneset of alternate servo tracks (e.g., the even servo tracks) in FIG. 5A,and the −PRBS is used in the other set of alternate servo tracks (i.e.,the odd servo tracks) interleaved with the first set. The first −PRBS ineach odd servo track is offset from the +PRBS in each even servo trackby a fixed number of bits. Thus the PRBS used in any track (a firsttrack) is the inverse of the PRBS used in a second track immediatelyadjacent the first track. This allows the even and odd tracks to beuniquely identified. The STM field utilizes the uniqueness of thesealternating tracks to resolve the offset that occurs between adjacenttracks.

In each track there is a cyclic shift between the leading PRBS used forthe STM field and the following PRBS used for the PES field. The cyclicshift is incremented as the servo track number increases. Between theSTM field and the PES field is a cyclic discontinuity which results fromthe break in the PRBS created by incrementing the cyclic shifts in thePES field. The STM field and the PES field taken together comprise theTrack ID (TID) field.

Before and after each PRBS field is a cyclic pad which is part of theperiod of the PRBS. The output of the correlator is valid only for thelength of the two cyclic pads combined with the length of the extra PRBSrequired to obtain the offset between adjacent tracks. The longer thecyclic pad, the longer the output of the correlator remains valid.

FIG. 6 is a block diagram of the servo decoder 601 that replaces theprior art STM decoder 400, TID decoder 402 and PES decoder 403 (FIG. 3).The read-back signal from R/W Electronics 113 (FIG. 3) is input at 166to the +PRBS Correlator 602 which is matched to the +PRBS. When thecorrelator 602 matches a +PRBS it produces a positive dipulse. When thecorrelator 602 receives a −PRBS input it produces a negative dipulse.The output of correlator 602 is input to the +Sync Correlator 604 andthe −Sync Correlator 605. The sync correlators 604, 605 act as matchedfilters for the dipulse outputs from +PRBS Correlator 602 and producesignal peaks at the zero-crossings of the dipulse outputs. The +SyncCorrelator 604 produces a positive peak at the zero-crossing of the+PRBS dipulse response while the −Sync Correlator 605 produces apositive peak at the zero-crossing of the −PRBS dipulse response. Thusthe +Sync Correlator 604 uniquely identifies even tracks that have apositive dipulse response while the −Sync Correlator 605 uniquelyidentifies odd tracks that have a negative dipulse response. The outputsof Sync Correlators 604, 605 feed into the Timing control block 610which generates windowing for the TID Decoder 620 and the PES Decoder630. The TID Decoder 620 calculates the integer portion of the headposition and the PES Decoder 630 calculates the fractional portion ofthe head position.

FIG. 7 shows the typical correlator dipulse output for the STM field asthe head moves from Track N to adjacent Track N+1. Here the use ofalternating +PRBS patterns and −PRBS patterns for adjacent tracks, withthe −PRBS patterns having a fixed offset from the +PRBS patterns allowsthe even and odd tracks to be uniquely identified. The zero-crossing ofthe correlator 602 output is identified by finding the peak of theoutput of the Sync Correlators 604, 605. By uniquely identifying theeven and odd tracks, the offset on adjacent tracks can be taken intoconsideration by the Timing control block 610 which generates windowingfor the TID Decoder 620 and the PES Decoder 630. Thus the outputs of theSync Correlators 604, 605 for the PRBS in the STM field act as the STMdecoder.

FIG. 8 shows the typical correlator 602 dipulse output for the TID fieldas the head moves from Track N to Track N+2. The location of the dipulsezero-crossing indicates the circumferential position of the recordedPRBS. The cyclic shift between the leading and following PRBS in eachservo track changes by a fixed increment as the servo track numberincreases. The TID is resolved in TID Decoder 620 (FIG. 6) by comparingthe phase or cyclic shift of the second or following PRBS relative tothe first or leading PRBS. In the example of FIG. 8, the fixed offset ofthe first −PRBS field in Track N+1 is eight bits from the first +PRBSfield in Track N. Thus the first PRBS on Track N is at phase zero whilethe second PRBS on Track N is at phase zero plus X_(N). The phasedifference or cyclic shift X_(N) is directly proportional to the Track NTID that is output at 424 by TID Decoder 620. The first PRBS on TrackN+1 is at phase eight (because of the fixed offset of 8 bits) while thesecond PRBS on Track N is at phase eight plus X_(N+1). Again, the phasedifference or cyclic shift X_(N+1) is directly proportional to the TrackN+1 TID that is output at 424 by TID Decoder 620. The cyclic shift isincremented a fixed number of bits, e.g. one bit, with each track. Asshown in the example of FIG. 8, the cyclic shift is 1 bit per track, soX_(N) is zero, X_(N+1) is 1 bit, and X_(N+2) is 2 bits.

The fixed offset between adjacent tracks, which in this example iseight, is chosen to provide a suitable region over which the patternsare orthogonal. The optimum value depends on the length of the PRBS andthe density of the recorded pattern. In addition, the use of the +PRBSand the −PRBS for even and odd tracks is not required for decoding theTID field but is a useful property for decoding the STM field. Thus if aseparate mark or pattern is used for the STM, then the same PRBS can beused for all the tracks, provided there is the fixed circumferentialoffset between the leading PRBS in adjacent tracks.

FIG. 9 shows the typical correlator 602 dipulse output that is sent toPES Decoder 630 as the head moves from Track N to adjacent Track N+1 andreads the PES field. With the head positioned directly above Track N,the correlator 602 produces a strong output for the +PRBS in the PESfield. As the head moves from Track N to Track N+1 the correlator outputfor the +PRBS in the PES field in Track N decreases while the output forthe −PRBS in the PES field of Track N+1 increases. With the headpositioned midway between Track N and Track N+1 the output for the +PRBSand the −PRBS are equal. With the head positioned directly above TrackN+1, the correlator 602 produces a strong output for the −PRBS. In thepreferred embodiment, the magnitude of the output is measured as the sumof the absolute values of the correlator output within a specified timewindow. The contribution of an even track is measured in a windowcentered on the dipulse response of the +PRBS while the contribution ofan odd track is measured in a window centered on the dipulse response ofthe −PRBS. Again, the use of the +PRBS and the −PRBS patterns for evenand odd tracks is not required for decoding the PES but is a usefulproperty for decoding the STM field. Also, while in the preferredembodiment the PRBS used in the PES field in each track is identical tothe PRBS used in the STM field, the PES field can be a different PRBS,which would require its own correlator.

In the above description two PRBS are used in each track, with the firstor leading PRBS field being used to decode the STM and the second orfollowing PRBS field being used for the PES. However the invention isfully applicable to a system wherein a PRBS is used to encode either theSTM or the PES. For example, the system described above for using thePRBS in adjacent tracks to determine the PES can be used with otherconventional techniques for determining STM and TID, without the needfor a separate leading PRBS field in each track.

The pitch of the servo tracks is not necessarily the same as the pitchof the data tracks. The pitch for servo tracks is arbitrary butpreferably related to the effective width of the read transducer. If thetrack pitch is too small, then the effective read width will cover morethan two tracks. This degrades the performance of the system since thePRBS patterns are orthogonal for only two tracks in the radialdirection. If the servo track pitch is too large, then there will beregions in the radial direction for which the decoded PES changes littleor not at all.

Also, the area of the dipulse response for either the STM field or thePES field can be used for gain control on subsequent servo sectors.

As mentioned, the invention is not limited to magnetic recording harddisk drives, but is generally applicable to data recording systems thathave data recorded in adjacent data tracks that also include servoinformation for positioning the data recording head or transducer. Thesesystems include magnetic tape recording systems and optical diskrecording systems.

While the present invention has been particularly shown and describedwith reference to the preferred embodiments, it will be understood bythose skilled in the art that various changes in form and detail may bemade without departing from the spirit and scope of the invention.Accordingly, the disclosed invention is to be considered merely asillustrative and limited in scope only as specified in the appendedclaims.

1. A recording medium having a plurality of tracks containing servoposition information that form a servo pattern, the servo patterncomprising: in a first track, a first-track pseudo-noise (PN) sequence;and in a second track adjacent the first track, a second-track PNsequence offset along-the-track from the first-track PN sequence.
 2. Themedium of claim 1 wherein each of the first-track PN sequence and thesecond-track PN sequence is a pseudo-random binary sequence (PRBS). 3.The medium of claim 1 wherein the first-track PN sequence is differentfrom the second-track PN sequence.
 4. The medium of claim 1 wherein thesecond-track PN sequence is the first-track PN sequence inverted.
 5. Themedium of claim 1 further comprising a cyclic pad preceding each PNsequence, the cyclic pad being a fixed portion of a period of the PNsequence it precedes.
 6. The medium of claim 1 wherein the medium is amagnetic recording medium.
 7. The medium of claim 1 wherein the mediumis a recording disk.
 8. The medium of claim 1 wherein the first-track PNsequence is the leading PN sequence in the first track, wherein thesecond-track PN sequence is the leading PN sequence in the second track,and further comprising: in said first track, a PN sequence following theleading PN sequence; and in said second track, a PN sequence followingthe leading PN sequence.
 9. The medium of claim 8 wherein the leading PNsequence and following PN sequence in each track are identical, andfurther comprising: in said first track, a cyclic shift between theleading PN sequence and its following PN sequence; and in said secondtrack, a cyclic shift between the leading PN sequence and its followingPN sequence, the cyclic shift in the second track being different fromthe cyclic shift in the first track.
 10. The medium of claim 9 whereinthe medium has a plurality of alternating first and second tracks andwherein the cyclic shift in a track increases by a fixed incrementacross successive adjacent tracks.
 11. A magnetic recording disk havinga plurality of concentric circular tracks, each track having a pluralityof angularly-spaced servo sectors, the servo sectors in each track beingaligned generally circumferentially and radially with servo sectors inadjacent tracks, and wherein the servo sectors in radially-adjacenttracks form a servo pattern comprising: (a) in each of a first set ofalternating tracks, a leading pseudo-noise sequence with autocorrelationand capable of detection by a correlation filter (PN sequence) and afollowing PN sequence identical to the leading PN sequence; (b) in eachof a second set of alternating tracks interleaved with said first set, aleading PN sequence circumferentially offset from the leading PNsequence in said first set and a following PN sequence identical to theleading PN sequence; and wherein each track has a cyclic shift betweenthe leading and following PN sequence, the cyclic shift increasing by afixed increment across successive adjacent tracks.
 12. The disk of claim11 wherein each PN sequence is a pseudo-random binary sequence (PRBS).13. The disk of claim 11 wherein the PN sequence in the second set oftracks is different from the PN sequence in the first set of tracks. 14.The disk of claim 11 wherein the PN sequence in the second set of tracksis the inverted PN sequence in the first set of tracks.
 15. The disk ofclaim 11 further comprising a cyclic pad preceding each PN sequence, thecyclic pad being a fixed portion of a period of the PN sequence itprecedes.